Investigation of Vibrational Cooling in a Photoexcited Dichloro-Ruthenium Charge Transfer Complex Using Transient Electronic Absorption Spectroscopy

Abstract

Vibrational cooling of molecules in excited electronic states is ubiquitous in photochemical reactions in solution but challenging to infer in time-resolved electronic absorption experiments. We report the ultrafast photophysics of cis-dichlorobis(2,2′-bipyridine)ruthenium(II), Ru(bpy)₂Cl₂, a precursor molecule commonly utilized in synthetic modifications of a vast array of ruthenium complexes. Femtosecond time-resolved electronic absorption spectroscopy is used to track an ultrafast spectral narrowing of the excited-state absorptions at 475 nm (21,050 cm^–1) and 505 nm (19,800 cm^–1) due to the reduced ligand in the photoexcited molecular complex. These sharp features, which overlap with a broader ground-state bleach spanning 450 nm (22,220 cm^–1) to 600 nm (16,670 cm^–1), evolve rapidly with time constants of 16 ± 5, 15 ± 3, and 18 ± 2 ps, respectively, for ligand-centered (π → π*, 266 nm) and charge-transfer (t₂ → π*, 400 and 550 nm) excitations and constitute a direct signature of picosecond vibrational cooling.